Oracle学习笔记<<3>>

Oralce Architectural Components

Process Structure

Oracle takes advantage of various types of processes:

1)        User process: Started at the time a database user requests connection to the Oracle server

2)        Server process :Connects to the Oracle instance and is started when a user establishes a session

3)        Background process: Started when an Oracle instance is started

oracle有三种进程,分别为user process,server process,background process

 

User Process

1)        A program that request interaction with the Oracle server

2)        Must first establish a connection

3)        Does not interact directly with the Oracle server

Server Process

1)        A program that directly interacts with the Oracle server

2)        Fulfills call generated and returns results

3)        Can be dedicated or shared server

负责和客户端连接，并返回执行结果。有两种模式分为dedicated 和shared server模式

Background Process

Maintains and enforces relationships between physical and memory starututes:

1)        Mandatory background process :

DBWn     PMON    CKPT    LGWR  SMON

2)        Optional background process:

ARCn      LMDn    QMNn    CJQ0   LMON

RECO      Dnnn     LMS      Snnn   LCKn   Pnnn

Database Writer (DBWn)

DBWn writes when :

1)        Checkpoint occurs

2)        Dirty buffers reach threshold

3)        There are no free buffers

4)        Timeout occurs

5)        RAC ping request is made

6)        Tablespace OFFINE

7)        Tablespace READ ONLY

8)        Tablespace DROP OR TRUNCATE

9)        Tablespace BEGIN  BACKUP

 

DBWn 中的n 有20个 从DBW0到DBW9, 从DBWa 到DBWg

Log Writer(LGWR)

LGWR writes:

1)        At commit;

2)        When one-third full

3)        When there is 1MB of redo

4)        Every three seconds

5)        Before DBWn writes

System Monitor

Responsibilities:

1)        Instance recovery

Rolls forward changes in online redo log files

Opens database for user access

              Rolls back uncommitted transactions

2)        Coalesces free space

3)        Deallcoates temporary segments

Process Monitor

Cleans up after failed processes by :

1)        Rolling back the transaction

2)        Releasing locks

3)        Releasing other resources

4)        Releasing dead dispatchers

Checkpoint(CKPT)

Responsible for:

1)        Signaling DBWn at checkpoints

2)        Updating datafile header with checkpoing information

3)        Updating control files with checkpoint information

 

Archiver(ARCn)

1)        Optional background process

2)        Automatically archives online redo log file when ARCHIVELOG mode is set

3)        Preserves the record of all changes made to database